Wolf Crosses the Lake Superior Ice to Become Leader of the Pack

<b>FATHER FIGURE </b> Fifty-six percent of the genes on Isle Royale can be traced to the Old Gray Guy, center, who migrated to Lake Superior in 1997.Credit
John Vucetich

In Ontario, in the winter of 1997, a particularly virile male wolf stepped onto the ice of Lake Superior and headed toward Isle Royale, an island about 15 miles offshore. There he radically changed the genetic makeup of an isolated group of wolves that had lived there since the late 1940s.

Researchers, who for many years have been observing the Isle Royale packs and the moose they feed on, did not realize at first that he was an immigrant, but soon his appearance and behavior became impossible to ignore.

He was larger than most of the Isle Royale wolves, and was so strongly territorial that he completely displaced one of the four packs, driving it to extinction within two years of his arrival. His own pack grew to 10 wolves, the largest seen on the island in almost 20 years. As he aged, his fur grew paler, almost white, a phenomenon known in other wolves but never before seen in the Isle Royale animals.

“We don’t know of any other instance — except when they first came — of wolves crossing the ice,” said John A. Vucetich, the lead author of a study of the wolves published online in The Proceedings of the Royal Society B last week. “The entire population is descended from a single female.”

The wolf population on Isle Royale is small under any circumstances — there were only 16 wolves there last winter, and the average is about 23. But by 2002, the new wolf, now designated wolf No. 93, and his offspring made up five of the six breeders. In his eight years of breeding, he produced 34 pups, and those pups produced an additional 45 progeny.

By analyzing DNA found in blood from some captured animals and in scat from many more, the researchers were able to determine that by 2009, 56 percent of the genes in Isle Royale wolves could be traced to wolf No. 93, or the Old Gray Guy, as he became known.

An error has occurred. Please try again later.

You are already subscribed to this email.

Such a change is clear evidence for a large difference in fitness between the Old Gray Guy and the other wolves, and scientists expected that the introduction of such a an animal into a small inbred group would produce a sharp increase in population — what scientists call a genetic rescue.

But in this case, it did not happen. A co-author of the study, Rolf O. Peterson, a research professor at Michigan Technological University, said that the population of Isle Royale hangs on by a thread, as it has for decades. The average reproduction after the Old Gray Guy arrived was no different from before. Yet this does not mean that he had no effect.

“The simple interpretation is that genetic rescue doesn’t work,” said Dr. Vucetich, an assistant professor of wildlife ecology at Michigan Technological University. “But what happened here is that when the immigrant came in 1997, in the decade that followed, the moose population declined radically. It’s plausible that we didn’t see an effect because the wolves were suffering from some other trouble that disguised the benefit.”

That, said Dr. Vucetich, is “an important lesson all over the world. When you do genetic rescues and it looks like it didn’t work, there may be a beneficial effect that you don’t see because of some other environmental event.”

What if wolf No. 93 had never arrived? Dr. Vucetich said that it is impossible to know for sure, but the Isle Royale wolves might have disappeared completely. It may be that the Old Gray Guy arrived just in time.

A version of this article appears in print on April 5, 2011, on Page D3 of the New York edition with the headline: Wolf Crosses the Lake Superior Ice to Become Leader of the Pack. Order Reprints|Today's Paper|Subscribe